Original video: https://youtu.be/DXtRerLq6j8
CAM photosynthesis (Crassulacean Acid Metabolism) is a fascinating adaptation that involves a unique four-phase cycle. This video breaks down each phase, explaining what happens during the night and day in CAM plants like cacti and succulents. Learn how they capture, store, and utilise CO2 to maximize photosynthetic efficiency while minimising water loss. We'll also touch upon the roles of key enzymes like PEP carboxylase and Rubisco, and discuss the differences between constitutive and facultative CAM.
Video Transcript:
For CAM plants, the system scientists use is to divide it into four phases: phase one, phase two, phase three, and phase four. Phase one is during the night. Stomata are completely open, then CO2 is captured and converted to malate.
Just as it approaches daytime, during dawn, is phase number two, the transition from phase one to phase three. During the daytime, stomata are completely closed, but the Calvin cycle is happening as usual. From this, it will move back to phase one. The in-between phase is what we call phase four.
Why is this important? You learned this morning, in the morning, different sets of enzymes are activated. At night, different sets of enzymes are activated. Similar to C4 plants, during the day, what enzymes do you think need to get activated? We'll say at night first because it's phase number one. During the night, PEP carboxylase needs to work hard to capture and convert CO2 into malate.
Do you need other enzymes to work as much as this? Actually, you don't. During the nighttime, PEP carboxylase gets activated, and then malate will accumulate and be stored in the vacuole. That's why in the picture here, the vacuole looks so big.
The transient phases are phase number two and phase number four, to shift the metabolism in preparation for phase number three and phase number one. We'll talk now about phase number two. Because the whole night it has worked so hard to capture the carbon, change it to malate, in phase number two, just before daytime, Rubisco activity increases. Different enzymes are activated now.
At night, is it sensible to activate Rubisco? No point. If you remember, to activate Rubisco you need light. You need Rubisco activase to activate Rubisco itself. You need light for it. You need the ferredoxin-thioredoxin pathway to be activated.
This is light. Electrons move from one molecule to another. One guy gets oxidized, one guy gets reduced, and at the end, some chemical reaction can happen because the reduction of some amino acid residue, like serine, changes the conformation of the enzyme from activated to non-activated, or vice versa.
Rubisco, with the presence of light, gets activated, and now it can do the regular Calvin cycle. Do you think it's going to experience photorespiration in CAM plants? Compared to C4 plants, photorespiration has a slightly higher tendency to happen for the CAM plant because there is no physical separation. It is still within the same cell.
The concept of CAM plants is not fixed in stone. Some are called constitutive CAM plants, meaning that whether it likes it or not, it will follow the regular notion. At night, open stomata, do the carbon fixation. During the day, close stomata, do carbon assimilation. That's the traditional, conventional definition of the CAM plant. Constitutive CAM plants use the nocturnal uptake of CO2 at all times.
However, this is not set in stone. There are some CAM plants which are called facultative CAM, meaning that they can resort to the CAM pathway only when they are under water stress or intense sunlight radiation. If they feel happy, they are just regular C3. Meaning that when they have this, don't you think they will be subjected to photorespiration? Of course, yes, because they choose C3. When you choose C3, photorespiration will definitely happen. Why? Because photosystem photolysis immediately releases oxygen. This oxygen has to go somewhere. If the mitochondria are not taking it off, it will compete for the active site at Rubisco, which is within the chloroplast. Photorespiration will happen.
Regardless of this, whether it's constitutive, facultative, or having all four phases or maybe just three phases, the concept of carbon concentrating mechanism is still there. For the CAM plants, they just use time to help concentrate the CO2.
You might wonder, why don't plants make all the new enzymes altogether? Do not make Rubisco. Whoever wondered that, why plants do not just simply make a new enzyme that is completely blind to oxygen? Isn't it the definition of an enzyme that you learned, the activity is specific, lock and key. Now, with Rubisco, one lock, you can use two keys. Why are plants not doing it? It must stick to Rubisco.
I have a partial answer. Partly because Rubisco is a multimeric protein. It has many compartments to make one protein, the large subunit and the small subunits, come to attach together. The subunits do not entirely come from the nucleus. Some of these proteins actually come from the chloroplast. Then they fuse together, then you get your Rubisco. It's not like any other protein. Most of the proteins in plants, it's either completely nuclear protein or mitochondrial protein or chloroplastic protein. But, Rubisco is special, it requires different organelles just to make one of it.
And maybe you want to blame CO2 as well, because CO2 solubility depends on temperature. The higher the temperature, the less soluble CO2 in the solution, and acidity. That's why you need magnesium. There are many things at play, so it's not easy for the plants to, because if you just change one enzyme, the rest of it needs to change as well, all the reaction cascades back, and it's going to take a very long time to go.
So, the plants see that the quickest way to do this, we just do the CCM, carbon concentrating mechanism, it works just fine. You only need 70 days to harvest corn. What about pineapple, it, CAM, is it fast or slow to harvest? Can you get it in 70 days? That's a key difference. Even though it looks similar, CAM and C4, but CAM is actually rather slow because of this nocturnal carbon fixation solely. Unlike C4 plants, as long as there is light, it can do the carbon fixation and assimilation throughout the day, no problem.
Keywords: Photosynthesis, CAM, Four phases, Plant metabolism, Day, Night
Watch full video: https://youtu.be/ckiJ_go8IaU
Reference book: Plant Physiology and Development 7th Edition
by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger
Attribution 4.0 International — CC BY 4.0 - Creative Commons
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